Integrated Circuit Package and a Method for Forming an Integrated Circuit Package

ABSTRACT

A method of forming an integrated circuit package, such as a Flip Chip package, in which a void is provided in the underfill material in the central region of the package between the chip or die and the substrate on which the chip or die is mounted. This reduces delamination of the package as a result of moisture.

This application is a continuation of co-pending International Application No. PCT/SG2005/000270, filed Aug. 4, 2005, which designated the United States and was published in English of which application is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method of manufacturing an integrated circuit package, and to an integrated circuit package manufactured according to the method. In particular, embodiments of the invention relate to a Flip Chip (FC) or Direct Chip Attach (DCA) package in which the chip is attached directly to the substrate, board or carrier by conductive bumps.

BACKGROUND

In integrated circuit packages, the electronic components are mounted on a substrate, circuit board or carrier. The electrical connection between the components and the substrate can be achieved through wire bonds, or through connecting bumps, such as solder bumps. The chip, substrate and interconnection are typically encapsulated to produce the final package.

In Flip Chip packages, also known as Direct Chip Attach packages, the electronic components are directly connected to the substrate, circuit board or carrier by means of conductive bumps on the chip bond pads. To achieve this, the chip is normally turned face down (i.e., flipped) for connection to the substrate.

As well as providing the direct electrical connection between the chip and substrate, the bump provides a thermally conductive path for carrying heat from the chip to the substrate, as well as providing part of the mechanical mounting of the die to the substrate. The bump also acts to space the chip from the substrate, thereby preventing electrical contact between the chip and the substrate.

In Flip Chip assemblies, a non-conductive underfill adhesive is provided under the chip filling the void between the chip and the substrate. This underfill layer acts to protect the bumps from moisture or other environmental hazards, and provides additional mechanical strength to the assembly. Importantly, the underfill mechanically locks together the chip and substrate so that differences in the thermal expansion of the chip and substrate do not break or damage the electrical connection of the bumps. Accordingly, the underfill layer is important in maintaining the integrity of the package, and especially to maintain the integrity of the package despite the ambient conditions.

One problem with known Flip Chip packages is their susceptibility to moisture attack, and in particular “popcorn-delamination” whereby moisture can cause the delamination of the chip, underfill layer and substrate. Such delamination can result in the failure of the electrical connection between the chip and substrate.

It is currently considered that the moisture performance of packages is dependent upon the materials used for the flux, underfill and mold compounds, and to the adhesion between the die and substrate. However, there is a limit in the improvements that can be achieved merely by selection of materials and improvement of the adhesion.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method for producing an integrated circuit package is provided in which the underfill material between a chip or die and substrate, circuit board or carrier on which it is mounted includes a void underneath a generally central portion of the chip.

The present inventors have found that popcorn-delamination in Flip Chip packages initiates from the center of the package, and progresses towards the edge of the package. By providing an area around the center of the package in which no underfill material is present, the possibility for delamination between the chip and the underfill layer in this central region can be avoided. By elimination of the initiation site, it is believed that the moisture performance of the package can be improved.

It has been found that the absence of underfill material in the center region of the package does not result in a significant decrease in the performance of the functions of the underfill layer, in particular that this does not significantly decrease the mechanical strength of the assembly, or the ability to compensate for thermal expansion differences between the chip and substrate. Further, since the underfill material can still encapsulate the bumps connecting the chip and substrate, the underfill material is still able to protect the bumps from moisture or other environmental hazards.

The central void can be provided using a seal pass, in which the underfill material is provided along each side of the chip, entrapping the central void. In one example, the seal pass is carried out quickly so that a volume of air or other ambient gas is trapped under the center of the chip preventing the underfill material from flowing into the central void. The fast seal-pass is important when the underfill material has a low viscosity. Alternatively or additionally, the underfill material may have a high viscosity, for example, a viscosity of at least 50 Pa.s. In this case, a seal-pass method is again used to deposit the underfill material along each side of the chip. However, the use of a material of high viscosity minimizes the flow rate of the material, and therefore reduces the need for the seal-pass to be carried out at high speed. By using an underfill material with a high filler loading, for example, with a filler loading of at least 75%, the material will have a sufficiently high viscosity to avoid the need for the seal-pass to be completed quickly. An additional advantage of using a material with a high filler loading is that the material will have improved moisture characteristics.

A suitable underfill material is an epoxy based material. This may be loaded with a filler material such as silica.

By providing a central void in the underfill material, it is possible to reduce the amount of underfill material required, and therefore reduce material costs for production of the integrated circuit package. Further, since less material is required to be provided and cured between the chip and underlying substrate, the time for forming the underfill material layer can be reduced. In particular, this can be achieved as there is no requirement to provide underfill material beneath the entire chip surface, which can reduce the time required to deposit the material, and less curing is required since the amount of underfill material to be cured is reduced. This can also enable the use of a fast-seal pass.

According to a second aspect of the present invention, there is provided an integrated circuit package comprising a chip or die bonded to a substrate, carrier or circuit board, and including an underfill layer between the chip or die and the substrate, carrier or circuit board, in which a void is provided containing no underfill material underneath a generally central part of the chip.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a scanning acoustic microscopy image of a known Flip Chip package after moisture tests, showing popcorn delamination;

FIG. 2 shows a cross-section of a conventional Flip Chip package;

FIG. 3 shows a partially complete view illustrating an underfill process for a known Flip Chip package with L-line dispensing;

FIG. 4 shows a cross-section of a Flip Chip package made in accordance with the method of an embodiment of the present invention; and

FIG. 5 shows an underfill according to an embodiment of the present invention using scan pass dispensing.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A conventional Flip Chip package will be described with respect to FIGS. 2 and 3.

As shown in the cross-section of FIG. 2, a chip 2 is mounted on a substrate, carrier or circuit board 4 by means of conductive bumps 6. In a typical example, the bumps 6 are formed by sputtering, plating or printing a solderable material on the chip 2, and these are connected and soldered to chip bond pads provided on the substrate 4.

A non-conductive underfill layer 8 is provided under the chip 2. As can be seen in FIG. 2, the underfill material surrounds the bumps 6, and fills the entire region underneath the chip between the chip and the substrate. Typically the underfill extends beyond the outer periphery of the chip. It is typical to provide the underfill material by dispensing the material along one or two sides of the periphery of the chip, allowing the material to flow under the chip to fill the area between the chip and substrate. Where material is deposited along one side only, this is known as “single- line” dispensing. Where material is deposited along two adjacent sides, this is known as “L-line dispensing”. FIG. 3 shows the situation in which the material is dispensed from the top and left hand sides of the chip, the flow covering the upper left hand corner of the chip as shown in the Figure. When the material has been dispensed underneath the chip, the material is cured.

A problem with conventional packages formed in this way is the popcorn-delamination that may occur if moisture enters the package. FIG. 1 shows a scanning acoustic microscopy image of a conventional package after moisture tests. The areas of failure can be seen as the light areas around the center of the package. The failure initiates from the center of the package and progresses outwardly towards the edge.

According to the present invention, a void 12 (See FIG. 4) is provided underneath the chip 2 between the chip and the substrate, this void including no underfill material.

The central void 12 can be achieved by dispensing underfill material around the entire periphery of the chip. By rapidly depositing the material along all sides of the chip, a volume of air or other ambient gas is sealed in the central void defined by the material. This entrapped gas prevents the further inward flow of underfill material and therefore ensures that the central void remains. By using underfill material having a high viscosity, and supplying this around the entire circumference of the chip, the material can be deposited more slowly whilst retaining the central void.

It has been found that this intentional central void removes the interfaces between the substrate 4 and the underfill material 8, and between the underfill material 8 and chip 2 in the central region of the package. By removing the possible interfaces, it is not possible for delamination to occur between the interfaces in this area. As it has been found that the main problem associated with delamination is initiated in this central region, it will be appreciated that removal of the interfaces in this region removes this initiation site.

By introducing the filler material around the entire periphery of the chip and retaining a central void, less underfill material is used compared to prior art packages, and the material can be dispensed and cured more quickly, leading to reduction in manufacturing time and manufacturing materials. This in turn leads to a reduced manufacturing cost.

The inventors have found that retaining underfill material around the central void achieves the advantages of reducing the strain on the bumps to a level consistent with that achieved in existing Flip Chip packages, the strain being much less than associated with packages not having any underfill layer. The results of the strain on the outermost bumps, where the strain is the largest, are given below for an existing chip including underfill, a chip with no underfill layer, and for an embodiment of the present invention.

Although only a single embodiment of the invention has been described in detail, many modifications may be made to it within the scope of the invention as will be clear to the skilled reader. 

1. A method of forming an integrated circuit package, the method comprising: providing a die having a central region and a periphery surrounding the central region; providing a substrate to which the die is to be mounted; attaching the die to the substrate using a plurality of bumps, the die being spaced from the substrate; and, providing an underfill material in the space between the die and the substrate, leaving a void containing no underfill material between the die and the substrate in the central region of the die.
 2. The method according to claim 1, wherein providing the underfill material comprises introducing the underfill material from around the periphery of the die, the material not flowing into the space between the substrate and die below the central region of the die.
 3. The method according to claim 1, wherein the underfill material has a high viscosity.
 4. The method according to claim 2, wherein the underfill material has a viscosity of at least 50 Pa.s.
 5. The method according to claim 1, further comprising solidifying or curing the underfill material other than below the central region of the die.
 6. The method according to claim 1, wherein the underfill material has a high filler loading.
 7. The method according to claim 6, wherein the filler loading of the underfill material is at least 75%.
 8. The method according to claim 1, in which the underfill material comprises an epoxy based material.
 9. The method according to claim 1, wherein the underfill material encapsulates at least some of the bumps.
 10. The method according to claim 1, further comprising encapsulating the die, substrate and underfill material.
 11. An integrated circuit package comprising: a substrate; a die having a central region and a periphery surrounding the central region, the die being attached to and spaced from the substrate; and an underfill material between the substrate and the die, the underfill material not being provided in a region below the central region of the die thereby leaving a void below the central region.
 12. The integrated circuit package according to claim 11, wherein the underfill material comprises an epoxy based material.
 13. The integrated circuit package according to claim 11, wherein the underfill material has a high viscosity.
 14. The integrated circuit package according to claim 13, wherein the underfill material has a viscosity of at least 50 Pa.s.
 15. The integrated circuit package according to claim 11, wherein the underfill material has a high filler loading.
 16. The integrated circuit package according to claim 15, wherein the underfill material has a filler loading of at least 75%.
 17. The integrated circuit package according to claim 11, further comprising a plurality of bumps between the substrate and the die.
 18. The integrated circuit package according to claim 17, wherein the substrate comprises a wafer.
 19. The integrated circuit package according to claim 17, wherein the substrate comprises a circuit board.
 20. The integrated circuit package according to claim 17, wherein the substrate comprises a carrier.
 21. The integrated circuit package according to claim 17, further comprising an encapsulation layer encapsulating at least a portion of the die, underfill material, bumps and substrate. 